Add gitsigns via vim.pack, track the pack lockfile

Restores plugin management after the v0.12 config rewrite dropped the
earlier `vim.pack.add` calls. Plugins now live in `config/plugins/`,
one file per plugin, so adding the next is a single require.

Sets `<leader>` to Space for the gitsigns hunk mappings, and pins
'signcolumn' open so git and diagnostic signs don't shift text.

The lockfile moves into the repo and is symlinked by bootstrap.sh, as
the vim.pack docs recommend, giving reproducible plugin revisions
across machines and a rollback path via git. vim.pack writes it with a
truncating open, so it follows the symlink rather than replacing it.

Drops the orphaned nvim-tree entry, unreferenced since the rewrite.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neovim/nvim-pack-lock.json

@@ -0,0 +1,8 @@
+{
+  "plugins": {
+    "gitsigns.nvim": {
+      "rev": "31d6fb2d618bca1482b9f274751ead5f03461408",
+      "src": "https://github.com/lewis6991/gitsigns.nvim"
+    }
+  }
+}